Ionisation Enthalpy, Oxidation States — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on ionisation enthalpy and oxidation states of transition elements, a multi-pronged strategy is essential. Firstly, memorize the electronic configurations of the 3d series elements (Sc to Zn), paying special attention to exceptions like Cr ($3d^54s^1$) and Cu ($3d^{10}4s^1$).

This is foundational. Secondly, understand the general trends of ionisation enthalpy across a period and down a group, but more importantly, focus on the reasons for irregularities and exceptions.

For example, why are $IE_1$ of Cr and Cu higher? Why is $IE_3$ of Mn exceptionally high? Why does lanthanoid contraction affect 5d elements' IE? Thirdly, for oxidation states, learn the common and maximum oxidation states for each 3d element.

Pay attention to which oxidation states are stable under specific conditions (e.g., $Mn^{2+}$ is stable due to $d^5$, $Zn^{2+}$ due to $d^{10}$). Fourthly, understand the underlying principles for variable oxidation states (small energy difference between $ns$ and $(n-1)d$ orbitals) and factors stabilizing higher oxidation states (electronegative ligands).

Finally, practice questions involving disproportionation reactions and their explanations. When solving MCQs, carefully read the question to identify if it asks for a general trend, an exception, or a specific reason.

Eliminate options that contradict fundamental principles or known exceptions. For numerical problems (less common for this specific subtopic, but possible for related calculations), ensure correct application of formulas and units.